1) Technical Field of the Invention
The present invention relates to a semiconductor optical device such as a semiconductor optical modulating device (semiconductor optical modulator) and a semiconductor optical detecting device (semiconductor optical detector), and in particular, to such a semiconductor optical device used for an optical communication system of 1.3 xcexcm-band.
2) Description of Related Arts
FIG. 6 is a schematic perspective view of a conventional semiconductor light modulating device denoted by reference numeral 400. The semiconductor light modulating device 400 includes a substrate 1 of InP, having an upper portion formed in a ridge configuration. Provided on the ridge portion are a light absorbing layer 2 and a pair of optical waveguide layers 13, so that the light absorbing layer 2 is sandwiched by the pair of the optical waveguide layers 13. The light absorbing layer 2 may be formed as a bulk semiconductor layer or a multi-quantum-well layer (referred to as a xe2x80x9cMQW layerxe2x80x9d) of material such as InGaAsP and InGaAlAs. On the other hand, the optical waveguide layer 13 is formed as a bulk semiconductor layer of InGaAsP. Successively deposited on the light absorbing layer 2 and the optical waveguide layers 13 are a cladding layer 4 of p-InP and a contact layer 5 of p-InGaAsP. Further, deposited on a top surface of the contact layer 5 and a bottom surface of the substrate 1 are an anode electrode 6 of Ti/Au and a cathode electrode 7, respectively.
In the semiconductor optical modulating device 400 of FIG. 6, an incident light 51, which is a continuous wave (referred to simply as xe2x80x9cCWxe2x80x9d) of 1.3 xcexcm-band, enters the optical waveguide layer 13, and is guided into the light absorbing layer 2.
A reverse biasing voltage is applied between the anode and cathode electrodes 6, 7 as a modulating electric signal of high frequency. The modulating electric signal causes the light absorbing layer 2 to absorb the CW light due to the Quantum Confined Stark effect or the Franz-Keldysh effect.
Thus, the outgoing light 52 from the other optical waveguide layer 13 has an amplitude and/or a phase modulated in the light absorbing layer 2.
FIG. 7 is an energy-band diagram of a region including and adjacent to the optical waveguide layer 13 in a cross section taken along the line VIIxe2x80x94VII of FIG. 6, illustrating the band gap of the optical waveguide layer 13 formed as the bulk semiconductor layer between the substrate 1 and the cladding layer 4. The optical waveguide layer 13 has a composition ratio of InGaAsP selected so that it has a band gap energy corresponding to photon energy of light having a predetermined wavelength (referred to simply as a xe2x80x9cwavelength xcexg xe2x80x9d), which is 1.1 xcexcm or shorter, for example. The wavelength xcexg (1.1 xcexcm) of the optical waveguide layer 13 is shorter than the wavelength of the CW light (1.3 xcexcm) so as to reduce absorption of the CW light within the optical waveguide layer 13.
However, when the optical waveguide layer 13 of InGaAsP has a wavelength xcexg of 1.1 xcexcm, it has a refractive index of approximately 3.30. Meanwhile, the substrate 1 of InP and the cladding layer 4 of InP have refractive indices of approximately 3.21. Thus, there exists a small difference (0.09) of the refractive indices between the substrate 1 and the cladding layer 4, and the optical waveguide layer 13.
In order to confine the incident CW light in the optical waveguide layer 13 in an efficient manner as realized by a commercially available semiconductor modulating device of 1.55 xcexcm-band, the difference of the refractive indices therebetween should be 0.15 or more.
Although the longer wavelength xcexg of the optical waveguide layer 13 can improve the efficiency of the optical confinement, it disadvantageously causes the optical waveguide layer 13 to absorb the incident light more.
Therefore, one of the embodiments of the present invention is to provide a semiconductor optical device, in which the light of 1.3 xcexcm-band is less absorbed and confined in a more efficient manner in the optical waveguide layer.
One of the embodiments of the present invention has an object to provide a semiconductor optical device including a substrate, an optical waveguide layer formed on the substrate as a multi-quantum-well layer having well and barrier layers. The semiconductor optical device also includes a light absorbing layer formed on the substrate and adjacent to the optical waveguide layer so that an incident light having an incident wavelength xcexLD through the optical waveguide layer is guided into the light absorbing layer. It also has a cladding layer formed both on the optical waveguide layer and the light absorbing layer, and a pair of electrodes formed so as to sandwich the optical absorbing layer. Each of the well layers has a wavelength xcexg longer than the incident wavelength xcexLD. Also, an effective transition energy gap between the lowest energy levels of a conduction band and a valence band of the optical waveguide layer is larger than a photon energy of the incident light.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the sprit and scope of the invention will become apparent to those skilled in the art from this detailed description.